This invention relates to methods of improving adhesively bonded aluminum structures, and to a method for inhibiting the conversion of aluminum oxide to aluminum hydroxide.
In the past, many processes have been described for treatment of aluminum surfaces in preparation for subsequent painting, adhesive bonding, and the like. The general intent of these preparation procedures is to modify the original surface as it comes from the mill so that it is (i) more suitable for developing strong bonds to polymeric materials and/or (ii) more environmentally stable against the effects of moisture and humidity.
The degree of success achieved to meet these goals varies considerably depending upon the process. For example, the Forest Products Laboratory (FPL) process, which consists of etching the aluminum in an aqueous sodium dichromate sulfuric acid solution, has been used extensively in the aircraft industry because it provides a rough surface oxide on the aluminum, which interlocks with the adhesive to form a strong bond, as shown by Venables et al. (J. D. Venables, D. K. McNamara, J. M. Chen, T. S. Sun, and R. L. Hopping, Appl. Surface Science 3, 88, 1979). Since the protective oxide is very thin (200-400 A), however, structures made using this process tend to be relatively unstable when brought into contact with moisture. On the other hand, chemical conversion coatings, which usually involve treatment with a highly acid solution containing one or more species of chromate, provide a form of chromium hydroxide surface which is highly resistant to environmental moisture and humidity, but which bonds with only marginal strength to polymeric coatings because of its relative smoothness. Thus, conversion coatings are successfully used under paints, but are not generally used for adhesively bonded structural components requiring a great deal of bond strength. In addition, environmental concerns have severely restricted the discharge of waste chemicals into waterways and sewage treatment facilities. Such restrictions normally require complete after-treatment of spent chemical conversion coating solutions, thus greatly increasing costs.
Another process, that of phosphoric acid anodization (PAA), discussed in U.S. Pat. No. 4,085,012 and 4,127,451, improves upon this situation by providing an oxide film on aluminum, which is both rougher and, at the same time, considerably thicker (2000-4000 A) than that provided by the FPL process. Since the effect is to provide good bond strength and also improved environmental stability, the process has become popular for use in the aircraft industry.
For those materials prepared by the FPL or PAA process for use in adhesively bonded structural aircraft components, the aluminum oxide coating formed during the treatment serves as a barrier, resisting corrosion and subsequent delamination of the structure. The effectiveness of this barrier, however, is compromised by the fact that when the aluminum oxide is subjected to a humid or moist environment, it transforms with time to a hydrated material known as pseudo-boehmite whose formula is approximately Al.sub.2 O.sub.3.2H.sub.2 O. This transformation is undesirable because (i) the change in chemistry is accompanied by a drastic change in shape or morphology, and (ii) pseudo-boehmite is not as tenaciously adherent to aluminum as is aluminum oxide. The effect of the transformation from aluminum oxide to aluminum hydroxide therefore is to destroy the interlocking grip between the oxide and adhesive (or primer) as the morphology changes, and to further weaken the bond strength due to the inherent lack of adequate interfacial strength between aluminum and pseudo-boehmite. Thus, although there may be differences in degree between the environmental stability of aluminum surfaces prepared by, for example, the FPL and PAA processes, they are subject to the same eventual degradation mechanism since the surface aluminum oxide in both cases hydrates in the presence of water.
The degradation of surface aluminum oxide structures is especially critical in composite assemblies used in the aircraft industry and elsewhere. For example, in aircraft construction, wing structures frequently utilize adhesively bonded aluminum materials which are subjected to temperature extremes varying from arctic cold to tropical heat, as well as exposure to varying atmospheric pressures and moisture content. To ensure safety, and to avoid failure of aircraft structures, bonded metal-to-metal and composite assemblies must be able to withstand extreme environmental conditions, as well as conditions of extreme flex and stress. A particular concern is resistance to corrosion and delamination of adhesively bonded structures under conditions of high humidity.